
Asymptotic expension of small argument (see [1]}) :
\(\forall n\in \mathbb {N}^{*},\quad I_{n}(\lambda )\underset{\lambda \rightarrow 0}{\sim }\frac{\left(\frac{1}{2}\lambda \right)^{n}}{\Gamma (n+1)}\quad \mbox{ and }\quad K_{n}(\lambda )\underset{\lambda \rightarrow 0}{\sim }\frac{\Gamma (n)}{2\left(\frac{1}{2}\lambda \right)^{n}}.\) 
